1. Field of the invention
This invention relates to a process sequence for the fractional distillation of light end components such as those which might be produced by steam cracking, catalytic cracking and coking and, more particularly, to a process sequence for separating propylene from a mixture of light end components which eliminates the need for a depropanizer unit.
2. Description of the prior art
Reaction conditions for steam cracking are selected to maximize the production of light olefins. Typically, cracking is practiced at a weight ratio of 0.3:1.0 of steam to hydrocarbon with the reactor coil outlet at 760.degree.-870.degree. C., and slightly above 100 kPa (atmospheric) pressure.
The type of feedstocks and the reaction conditions determine the mix of products produced. Many steam crackers operate on light paraffin feeds consisting of ethane and propane and the like. However, a significant amount of steam cracking capacity operates on feedstocks which contain propane and heavier compounds. Steam cracking such feedstocks tends to produce significant amounts of propylene, propane, butenes, and butadiene. It is in the separation of steam cracked products from these feedstocks that this invention has its application.
During steam cracking, cracked gases emerging from the reactors are rapidly quenched to arrest undesirable secondary reactions which tend to destroy light olefins. The cooled gases are subsequently compressed and separated to recover the various olefins.
The recovery of the various olefin products is usually carried out by fractional distillation using a series of distillation steps to separate out the various components. Generally, one of two basic flow sequences is used. The two sequences are usually denominated as the front-end depropanizer sequence, commonly referred to as `front-end deprop`, or the front-end demethanizer sequence, commonly referred to as `front-end demeth`.
In either sequence, gases leaving the cracking ovens are quenched, compressed, have their acid gas removed, and are dried. At this point the two flow sequences diverge. In the front-end depropanizer sequence the gases, which contain hydrocarbons having from one to five or more carbon atoms per molecule (C1 to C5+) next enter a depropanizer. The heavy ends exiting the depropanizer consist of C4 to C5+ compounds. These are routed to a debutanizer where the C4's and lighter species are taken over the top with the rest of the feed leaving as bottoms which can be used for gasoline or other chemical recovery. The tops of the depropanizer containing C1 to C3 compounds are fed to an acetylene hydrogenation unit then a demethanizer system where the methane and any remaining hydrogen are removed as an overhead. The heavy ends exiting the demethanizer system which contains C2 and C3 compounds are introduced into a deethanizer wherein C2 compounds are taken off the top and C3 compounds are taken from the bottom. The C2 species are, in turn, fed to a C2 splitter which produces ethylene as the light product and ethane as the heavy product. The C3 stream is fed to a C3 splitter which separates the C3 species, sending propylene to the top and propane to the bottom.
In the front-end demethanizer sequence the quenched, compressed acid-freed and dried gases containing C1 to C5+ compounds first enter a demethanizer system, where C1 and any hydrogen are removed. The heavy ends exiting the demethanizer system consists of C2 to C5+molecules. These are routed to a deethanizer where the C2 species are taken over the top and the C3 to C5+compounds leave as bottoms. The C2 species leaving the top of the deethanizer are fed to an acetylene hydrogenation or recovery unit, then to a C2 splitter which produces ethylene as the light product and ethane as the heavy product. The C3 to C5+stream leaving the bottom of the deethanizer is routed to a depropanizer which sends the C3 compounds overhead and the C4 to C5+components below. The C3 product is fed to a C3 hydrogenation unit to hydrogenate C3 acetylenes and dienes, then to a C3 splitter where it is separated into propylene at the top and propane at the bottom, while the C4 to C5+stream is fed to a debutanizer which produces C4 compounds at the top with the balance leaving the bottoms to be used for gasoline.
A considerable amount of work has been done on improving the basic process of separating the products of steam cracking. Much of the work on light ends fractionation has been concerned with the improvement of the various components of the process. Other improvements relate to improved computer control of the process. Progress has also been made in the optimum design and operation of the process through the use of improved physical property correlations. Although there have been improvements in the sophistication of the design of fractionation steps such as two-tower demethanizers, deethanizers, and depropanizers, heat-pumped towers, and improved separation efficiencies through the use of dephlegmators, the basic flow sequences as outlined above have remained essentially unchanged.
A shortcoming of the presently known flow sequences is that they invariably feature a depropanizer which serves to split the C3 and lighter compounds from the C4 and heavier compounds. In some situations, depending on the market values of the various products and on the particular circumstances of the processing facilities, it may be unnecessary and wasteful to separate the C3 and lighter fraction from the C4 fraction. Specifically, where the relative value of propylene is sufficiently high and the C4 value is low and/or available separation facilities so dictate, it would be more profitable to produce propylene in preference to a complete slate of products.
It would thus be desirable to have a flow sequence capable of preferentially producing propylene using less separation equipment.